This invention relates to a semiconductor laser device, and more particularly to a stripe-geometry double heterojunction laser device.
A multiple heterostructure semiconductor laser made of a semiconductor crystal such as (Al.Ga)As has a low threshold current value. To take a GaAs-GaAlAs double heterojunction laser as an example, it is capable of a highly efficient oscillation even at room temperature, and thus it has been used in optical fiber communication. In order to operate lasers of this type at the smallest possible current value and to oscillate them in a single transverse mode or in a state close thereto, various stripe geometry laser devices have been proposed and manufactured for experimental purposes. As the main stripe structures, there have been reported a proton bombardment type stripe semiconductor laser, a mesa type stripe semiconductor laser, a planar type stripe semiconductor laser, an electrode type stripe semiconductor laser, etc. In these stripe semiconductor lasers, higher-order transverse modes are prone to oscillate at a stripe width of 20 odd .mu.m or above, and hence, the stripe width is usually set at 10 .mu.m to 20 .mu.m or so.
In all the cases, however, there are common disadvantages as described below. In the lateral direction of a light emitting region (the direction being parallel to a p-n junction and perpendicular to the optic axis of output light), an optical amplification gain distribution owing to injection carriers has a gentle optical guiding, and only this optical guiding acts to confine laser light. Moreover, in the case of a stripe width of approximately 10 odd .mu.m at which the oscillation of lower-order transverse modes is possible, a bell-shaped carrier density profile is liable to be formed by the injection and has a negative optical guiding, and the carrier density profile at the center of the stripe is reduced by an intense induced emission of the laser beam, so that the optical amplification and guiding is weakened. Therefore, the transverse modes become unstable and easy to move rightwards and leftwards, with the result that the oscillation occurs in a heavy loss state and that a non-linear part (kink) appears in the current-optical output characteristic frequently with reproducibility. In consequence, the near-field pattern and the far-field pattern vary, and serious drawbacks such as lowering of the coupling efficiency with an optical fiber or the like external optical system, degradation of modulation characteristics and increase of noise are encountered.
As an expedient for eliminating the nonlinearity, it has been reported to make the stripe as narrow as several .mu.m or so and to effect an oscillation in the shape of a filament. With this expedient, however, the kink is not perfectly removed, but merely the output level of the kink is raised. Besides, this measure of narrowing the stripe is undesirable for semiconductor laser characteristics in that the threshold current density rises abruptly, that the external differentiation quantum efficiency lowers and that the laser beam intensity which can be produced without causing any optical damage in a reflection surface lowers.
In order to eliminate the disadvantages of the prior art stripe geometry semiconductor lasers set forth above, it is necessary to stabilize the optical guiding in the horizontal direction without making the stripe width very small. On the other hand, in an optical communication system in which semiconductor lasers are principally used, a semiconductor laser for a light source needs to have its dynamic characteristics stabilized. A semiconductor laser device in which relaxation oscillations which appear in the dynamic characteristics are suppressed and which exhibits a stable rectangular response waveform to a rectangular driving current waveform is required as the light source for optical communication.